diff --git a/lib/libc.c b/lib/libc.c
index 97565c31..2819468c 100644
--- a/lib/libc.c
+++ b/lib/libc.c
@@ -424,7 +424,7 @@ sprintf (char *str, char const* format, ...)
 int
 isatty (int fd)
 {
-  return ioctl (fd, TCGETS, 0) == -14; // MAGIC
+  return ioctl (fd, TCGETS, 0) & 0xf0;
 }
 
 #endif //!POSIX
diff --git a/src/posix.c b/src/posix.c
index 2e6b3a8c..f0a020d9 100644
--- a/src/posix.c
+++ b/src/posix.c
@@ -199,3 +199,9 @@ force_output (SCM p) ///((arity . n))
 {
   return cell_unspecified;
 }
+
+SCM
+isatty_p (SCM port)
+{
+  return isatty (VALUE (port)) ? cell_t : cell_f;
+}